Various printing systems are known for recording (printing) character (text) or graphic (picture) images on a recording medium (receiver) such as a paper or polymer sheet. Examples thereof are set forth in the following prior art.
U.S. Pat. No. 3,553,424 (Spaulding), issued Jan. 5, 1971, discloses heat stabilizing (fixing) of latent images on spectrally sensitized, i.e., photographic emulsion, printout paper, by passing the paper along a multi-zone heating surface segregated by groove and stepped portions for progressive heating, followed by cooling.
U.S. Pat. No. 4,703,331 (Stevens, Jr.), issued Oct. 27, 1987, discloses a spark jet printer with a plurality of spark jet units, each having a spring-fed consumable, solid ink electrode with an end adjacent the end of a fixed counter electrode energized for issuing an ink spark jet to form images on paper.
U.S. Pat. No. 3,862,394 (Lane, III), issued Jan. 21, 1975, discloses a thermal print head with a trifluoro methylene coated, triangular shaped aluminum substrate having an apex forming a print edge across which electrically insulated copper plated wires extend. The insulation and copper plate are removed from the wires at the apex to form resistance heaters thereat.
U.S. Pat. No. 4,689,639 (Kimura etal.), issued Aug. 25, 1987, discloses a thermal print head with heating elements in a groove or on an edge portion thereof to heat an ink film (donor web) for ink transfer to paper to form images thereon.
U.S. Pat. No. 4,691,210 (Nishiguchi etal.), issued Sep. 1, 1987, discloses a thermal print head for heat sensitive recording, with heating elements separated from an alumina ceramic substrate by a glaze layer, mainly of silica, about 35 to 50 microns (0.0014 to 0.002 inch) thick. The elements have an inner heat generating resistor layer of tantalum nitride or titanium oxide (TiO), and an outer layer of pairs of aluminum or gold conductor electrodes. The elements are overcoated by a protecting layer of tantalum pentoxide. The type of thermal recording effected with the print head is not indicated.
U.S. Pat. No. 4,194,108 (Nakajima et al.), issued Mar. 18, 1980, discloses a thermal print head with groove separated resistance heaters for forming images in thermally sensitive paper at 500 degrees C. under a 200 gram per sq. cm. (2.8 psi) force.
U.S. Pat. No. 4,170,728 (Flasck), issued Oct. 9, 1979, discloses a thermal print head having an apex edge with non-conductive cement coated, side by side bent wire threads forming resistance heaters protruding from side by side notches in a metal heat sink support body coated with an anodized oxide insulating layer. The apex edge is encapsulated in an insulating potting material acting as a heat sink. The protruding threads contact and heat a heat responsive recording sheet to form images therein.
U.S. Pat. No. 4,350,449 (Countryman et al.), issued Sep. 21, 1982, discloses a resistive ribbon thermal transfer print head with a row of side by side, spaced apart, electrodes for sliding pressure contact with a fusible (meltable) ink bearing resistive ribbon overlying conventional paper on a platen. Electrothermic printing of images is effected by transfer of melted ink from the ribbon to the paper under high print head force during movement of the head relative to the ribbon and paper. The electrodes, which are of unidentified material, are embedded in a thin insulating layer between plates of unidentified material, and have exposed, ribbon contacting electrode ends. The ribbon has an upper, print head contacting resistive layer, an intermediate conductive ground layer of aluminum with a thin insulating layer of aluminum oxide, and a lower ink layer. Electrode energizing resistively heats discrete ribbon areas to release ink for transfer to the paper. The ground layer provides a short current path from the electrodes through the resistive layer for localized heating of contiguous ink portions in the ink layer, with current return from the ground layer to ground via an element remote from the electrodes. For imaging thermally sensitive paper, the ink layer is omitted from the ribbon.
IBM Technical Disclosure Bulletin, Vol. 26, No. 10A, March 1984 (Fathergill et al.), discloses a flexible electrode, multi-layer print head used for resistance ribbon printing to generate high temperatures near the print electrode tips which follow the ribbon closely during printing. As the heat is produced in the ribbon, high temperatures in the head are not required. The head has a first compliant layer of silicone rubber supporting a second heat sink layer of vacuum deposited copper or aluminum, coated by a third heat resistant resin layer, carrying a fourth thermally conductive adhesive layer for adhering thereto a fifth tungsten electrode layer which is overcoated by a sixth heat resistant resin top layer. The second heat sink layer protects the head from injury from heat while not adding undue rigidity thereto.
U.S. Pat. No. 4,484,200 (Tabata et al.), issued Nov. 20, 1984, discloses a recording head with an electrically insulating, epoxy resin support containing a row of side by side, spaced apart, recording electrodes and a common opposed return electrode. The head contacts a moving ribbon bearing electroconductive heat transferable, wax based ink for transfer to paper by Joule heat generated in the ribbon by image delineating current applied by selected recording electrodes, with current return via the return electrode. The resin support of the head has a contact surface at which the adjacent ends of the electrodes contact the ribbon. A transverse groove in the contact surface between the recording electrode ends and the return electrode end prevents ink from adhering to the head during printing. An exemplified ribbon has a carbon black loaded polyvinyl butyral resin base layer coated with a carbon black containing wax of 60 degrees C. melting point.
Japanese Patent Laid-Open No. 99,162/87 (Mormose), dated May 8, 1987 (per English translation), discloses a four layer recording head that contacts the resistance layer of a heat transfer sheet (ribbon) having a fusible ink layer, for passing current to fuse the ink for thermoelectric transfer to a recording sheet as image forming dots. The head has a first substrate layer of mica ceramics supporting a second layer of a row of side by side, spaced apart, recording electrodes, e.g., tungsten wires, of 250 micron (0.01 inch) pitch (center to center electrode distance). The recording electrodes may be secured to the first layer by an adhesive, e.g., silicon dioxide. A third spacer layer of heat resisting resin, e.g., polyimide, of thickness close to the recording electrode pitch, e.g., a thickness of 150 microns (0.006 inch), separates the second layer of recording electrodes from a fourth common return electrode layer. The distance between the recording electrodes and return electrode, which determines the occurrence of cross talk and unequal size printed image dots, depends on the third spacer layer thickness accuracy, rendering irrelevant the first substrate layer thickness accuracy.
U.S. Pat. No. 4,684,960 (Nishiwaki), issued Aug. 4, 1987, discloses a print head with a row of side by side, spaced apart, alternating polarity, recording electrodes, e.g., of positive polarity, and return electrodes, e.g., of negative polarity, such as tungsten, molybdenum and/or manganese electrodes, i.e., metal electrodes of relatively low hardness, of 10 to 30 micron (0.0004 to 0.0012 inch) thickness, supported on a common ceramic substrate of alumina, forsterite, etc., such as of 0.5 to 3 mm (500 to 3,000 micron; 0.02 to 0.12 inch) thickness. The electrodes and substrate have end faces in a contact plane for sliding contact with an electrothermal ink bearing transfer film (resistive ribbon) overlying paper on the resilient surface of a platen, e.g., under a low contact pressure of 1.2 to 2.2 kg per sq. cm. (17 to 31 psi), for heat transfer of wax based ink from the resistive ribbon to the paper to form images thereon. The ribbon may have an electrically conductive (resistive) first contact layer of a carbon powder containing resin, an optional supporting second layer of polyethylene terephthalate, and a third ink layer of wax and a pigment or dye that is fusible at 60 degrees C. The contact plane of the electrode and substrate ends is at an acute angle to the plane of the substrate supported row of electrodes.
A second embodiment has a ribbon with a first contact layer of high electrical resistance, a second metal or carbon layer of low electrical resistance, an optional third tensile layer, and a fourth (wax based) ink layer. The print head has recording electrodes perpendicular to the ribbon, and return electrode spikes remote from the print head to pierce the ribbon for conductive contact with the second layer to complete the circuit. A third embodiment has a print head with a row of recording electrodes and an opposed common return electrode akin to Tabata et al. discussed above.
It is noted that a resistive ribbon thermal transfer print head has electrodes that supply current to a resistive ribbon to generate heat in the ribbon to heat the dye therein. On the other hand, a thermal print head has resistors that generate heat in the head for transfer to a donor web to heat the dye therein.
In resistive ribbon thermal printing, heat is generated in an electrically resistive ribbon bearing thermally transferable dye when current flows through the resistive layer and ground layer (return electrode) materials of the ribbon. This is commonly referred to as Joule heating. Current is supplied to the ribbon by a linear array of discrete, electrically conductive electrodes in the print head, i.e., a row of side by side, spaced apart, electrodes mechanically supported by a substrate. Modulated current is fed to the electrodes as current pulses via conductors.
The resistive ribbon typically has an upper base layer of electrically resistive polymer for contacting the electrodes, an intermediate electrically resistive ground layer of conductive material, e.g., aluminum, on which an electrically resistive oxide layer, e.g., aluminum oxide, forms (grows), and a lower layer of dye heatable to a transfer temperature for transfer to a receiver.
There are three primary resistances in the ribbon current flow path. The first is the "contact" resistance at the contact interface between the electrodes and ribbon. The second is the "bulk" resistance at the bulk (mass) of the base layer resistive polymer. The third is the "interface" resistance at the interface of the conductive ground layer, e.g., aluminum, and its resistive oxide layer. The heat generated at each of these resistances contributes to the transfer of dye from the ribbon to the receiver.
A high force is required at the contact interface of the print head and ribbon for good compliance therebetween. This force, plus the high temperature that can occur in printing, unless controlled, can damage the ribbon and limit the electrical energy supplied thereto by the electrodes, making the operation energy inefficient.
Image quality is affected by the temperature profile in the dye mass being transferred. This profile is adversely influenced by the significant energy lost by heat transfer from the ribbon to the electrodes which heats the electrodes. This heat is conducted away from the electrodes by the substrate at a rate determined by the thermal conductivity of the substrate, typically a ceramic material such as steatite, alumina or magnesia.
A low thermal conductivity substrate, typically of 2 to 20 W/m.C (watts per meter per degree C.), such as steatite or alumina (95.0% purity) is normally used. This limits the operation to slow printing speeds and character (text) image production. If operated at faster printing speeds or for producing near-photographic (picture) images, dye trails (bleeding) and ribbon damage can occur due to slow electrode cool down.
Use of a substrate of high thermal conductivity, typically of 20 to 80 W/m.C., or higher, such as alumina (at least 99% purity) or magnesia, rapidly conducts heat away from the electrodes for fast electrode cool down, but can deprive the dye in the ribbon of the heat needed to transfer a proper dye amount to the receiver, particularly if the substrate has an exposed end face (contact face) in sliding contact with the resistive ribbon.
It is desirable to provide a print head having electrodes supported by a substrate for resistive ribbon thermal transfer printing, with means to control the electrode temperature.